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The fish that have bellies full of mice – but we don’t know how

Don’t go near the water

Jouan & Rius/NaturePL

By Robin Wylie

It’s a cat-and-mouse tale with a difference. The lesser salmon catfish has been found feasting on mice. But how does it catch them?

Some catfish are known to ambush unwary pigeons at the water’s edge, giving them the nickname “freshwater killer whales”. But the lesser salmon catfish might just be an opportunist, gobbling up animals when they drown.

A survey of 18 lesser salmon catfish (Neoarius graeffei) from Ashburton river in northern Australia, suggests the fish can consume large quantities of small land animals when given the chance — almost half of the catfish had mice in their bellies.

“That is a lot, and a rare finding,” says Peter Lisi, an aquatic ecologist at the University of Wisconsin-Madison.

The stomachs of some catfish contained as much as 95 per cent small mammals, with two fish having three animals each in their stomachs.

Lesser salmon catfish can grow to half a metre long and weigh up to 1.5 kilograms. They are a common species in dryland rivers of north-western Australia, so their diet is important to understanding the local ecosystems.

They were thought to feed mainly on aquatic invertebrates and plants, with the occasional addition of fruit and terrestrial insects, especially during the floods in the wet season.

And though a few freshwater fish species are known to dine on land vertebrates — African tigerfish have been filmed plucking a swallow out of thin air, for example — it is rare for them to eat so many.

The catfish had been mostly eating spinifex hopping mice (Notomys alexis, pictured above), which are around 10 centimetres long. As their name suggests, the mice get around by jumping. There are no reports of these mice intentionally spending any time in the water.

But heavy rain might have a role to play. “These mice often live in small colonies within a single burrow system,” says Erin Kelly of the Centre for Fish and Fisheries Research at Murdoch University, Perth, who led the research, “so collapse or flooding of one or multiple burrow systems along the Ashburton river could have inadvertently introduced them into the water.”

“When several catfish are targeting mice all at once, it suggests that a large pulse of mice are entering the river,” says Lisi. “We still do not know how catfish gain access to mice or how often it occurs, or at what scale mice support river food webs. Because large fish often survive through feast and famine periods, big meals like this are ecologically relevant.”

If this is what is happening, the mice could be in greater danger as climate change kicks in.

“Climate projections for north-western Australia indicate that we’re going to see both longer periods of drought and more intense rainfall events,” Kelly says. “Changes in periods of flooding could possibly be altering the food web of these fish.”